Question about connecting rods in V-engine

[Adaware]

I'm in the prosess of designing a V8 engine but for estetical reasons i want to have the two cylinderbanks parallel to each other.
On a normal V engine there are two conrods sharing the same crankpin but the cylinders on each bank does not align when this is done. I've studied the fork an blade style of connecting rods and it seems a bit to complicated for me so i came up with my own design but i need a second opinion...do you thing it will work or will the whole thing break apart as soon as i pass 1000rpm?

Fork and blade:


My design with centerline drawn:

[dynosor]

The offset forces will produce a torque that tries to lean over and bend each rod, possibly overloading the bearings at their edges; possibly fatiguing the rods due to bending flexure.

You drew the rods 180 degrees apart with no risk of interference - you should ensure that they have sufficient clearance at the "kink" to prevent interference, possibly by having a longer straight section before the kink.

To answer these questions, make a scale drawing of your idea and calculate the force vectors - the longer the rods, the smaller the angular problem becomes - your rods seem too short to mount the pistons. The heavier the cross-section and the wider the bearings, the less of a risk you have, but the greater the inertial balance problems become.

[T.L.A.R. eng]

I will agree with dynosor on all points mentioned. Off set rods are not really a good idea in my opinion.
Some antique engines of old were intentionally built this way and one I saw the internals of, had an extreme kink or jog right at the big end journal with a straight rod beam to the piston. These were constant speed low rpm engines. It might have worked, but is it the right way?

Either way, I would think that off set rods would place extreme side loads on the piston and the rod bearing. I would think the friction and wear would be rather high not to mention the limiting structural integrity at higher rpms.

Knife and fork rods are not that hard to duplicate in miniature.

[Adaware]

The drawing is in scale and there is room for a piston there. If i use a knife and fork approach i doubt i can get the two rods to be equal weight so i might have to go for the conventional way afterall, but i have come up with a solution to make the engine symetrical on the outside. The up side with the conventional design is that i get 8 rods of the same design instead of 4 fork and, 4 blade types. Ufortunately i don't have advanced CAD programs with strenght calculations and so on, just the good old Autocad 2002

My goal is to have a high revving engine with a flat plane crank so rod strenght will be crucial.

[tstom]

In some racing engines we have rods that the beam is straight but the pin end isn't in the center of the piston and they work fine probably never more than .050" If it's just a model engine you could probably go more

Why not offset the cylinders but have the outside of the block symmetrical by just leaving some extra material on the offset ends How much would you need to offset to get paired rods?

[stevehuckss396]

Try going with the forked rod. It should be easy to mill the center out with a ball end mill. You will end up with a much better product in the end.

[dynosor]

Originally Posted by tstom Why not offset the cylinders but have the outside of the block symmetrical by just leaving some extra material on the offset ends How much would you need to offset to get paired rods?

Yes; or use the space to house cooling ducts or a timing chain or something...

[dynosor]

Originally Posted by stevehuckss396 Try going with the forked rod. It should be easy to mill the center out with a ball end mill. You will end up with a much better product in the end.

The biggest thing against the forked rod in my opinion, is the small size bolts required to hold on the end caps. I say no to anything smaller than an M3... If the engine is large enough, this factor drops away.

[dynosor]

Originally Posted by Adaware The drawing is in scale and there is room for a piston there.

From your image, the ratio of rod length to stroke seems to be 1.5:1. I would consider this the minimum just to reduce the side loads on the piston with a conventional rod.

If you lengthened the rod to inrease that ratio to 2:1, the thrust angle trying to kink the rod is reduced. I would consider going even longer than that. Yes, the rod becomes slimmer, but the angles are more favorable.

[Adaware]

Originally Posted by dynosor From your image, the ratio of rod length to stroke seems to be 1.5:1. I would consider this the minimum just to reduce the side loads on the piston with a conventional rod. If you lengthened the rod to inrease that ratio to 2:1, the thrust angle trying to kink the rod is reduced. I would consider going even longer than that. Yes, the rod becomes slimmer, but the angles are more favorable.

The CC distance on the rod is 59,5mm, the stroke will be 30mm so the factor is 2:1.

Originally Posted by tstom In some racing engines we have rods that the beam is straight but the pin end isn't in the center of the piston and they work fine probably never more than .050" If it's just a model engine you could probably go more Why not offset the cylinders but have the outside of the block symmetrical by just leaving some extra material on the offset ends How much would you need to offset to get paired rods?

I need to offset from 9-11mm, not decided yet because it depends on the rod width that i have not decided. Also i want the exhaust and inlet ports to align with each other because, it will make the intake alot easier to make. My sollution is to make the ports more oval to overlap the opposite port on the other head, difficult to explain with my limited english vocabulary but i think this can work. It looks good on the drawing anyway.

Originally Posted by dynosor The biggest thing against the forked rod in my opinion, is the small size bolts required to hold on the end caps. I say no to anything smaller than an M3... If the engine is large enough, this factor drops away.

I hate threads under M3 too, prefer M4 but i can't always be lucky


But the fork and blade type, how do they get equal weight on each rod when the fork type has alot more mass than the blade type?
Am i missing something obvious here maybe?

[dynosor]

[quote=Adaware;559263]
But the fork and blade type, how do they get equal weight on each rod when the fork type has alot more mass than the blade type?
quote]


The fork has two narrow bearings and the blade one wide bearing. The total bearing (and beam area) are similar, so the inertia is similar. The big problem is the small bolt size in the fork, holding the end-cap on. That, and the additional machining complexity of making more springy parts.

See how Amsbury did it for his V8 in the attached file. Zipped PDF

[Adaware]

Nice PDF there

I don't see this to be practical in my design since i will prove difficoult to drill an oil channel to the wristpin trough the fork style rod, i have decided to go with the conventional way after a little thinking but the engine will appear symmetrical. It's convenient to reduse the number of different parts in this engine so it will be a more realistic project. After all, with CNC machines it's the same job to make 4 or 20 parts when it it small scale like this.

But thanks alot for your help, it's always good response here